Generally, plasma arc cutting machines and plasma arc welding machines are arranged such that DC current from a plasma are power supply apparatus (hereinafter referred simply to power supply apparatus) is caused to flow between an electrode and a workpiece, which form together a plasma load, to thereby generate a plasma arc between them in order to cut or weld the workpiece.
The power supply apparatus has its positive output coupled to the workpiece, and has its negative output coupled to one end of an electrode of a plasma torch. The plasma torch electrode is a cylinder of a metal, such as copper, having a relatively high electrical conductivity. In the other end of the torch electrode, a refractory material, such as hafnium, tungsten, and zirconium, is embedded. The torch electrode is surrounded by a nozzle with a spacing disposed therebetween. The nozzle includes an opening at its tip end.
In a plasma are cutter, for example, while a plasma gas, such as compressed air, oxygen, argon, hydrogen, and nitrogen, is being fed into the space between the nozzle and the torch electrode, DC current from the power supply apparatus is applied between the torch electrode and a workpiece having a gap formed therebetween. A high voltage is applied between the electrode and the workpiece for a short time in the initial stage to generate a pilot are in the gap. The pilot arc induces a plasma arc between the electrode and the workpiece, and the generated plasma arc is used to cut the workpiece. The plasma arc is generated from the refractory material at the tip end of the electrode and is directed to the workpiece through the opening at the tip of the nozzle.
The refractory material as a source of a plasma are is consumed as it is operated, and when the amount of the refractory material decreases below a predetermined limit, no plasma arc can be generated. In other words, the life of that torch electrode comes to the end. Then, a new electrode is substituted.
It has been considered that the amount of refractory material of the electrode consumed is smaller as the amount of ripples contained in the DC current applied between the electrode and a workpiece is smaller. Accordingly, a constant current power supply apparatus used to be employed.
In a constant current power supply apparatus, AC power is rectified and smoothed into DC power, the DC power is converted to high-frequency power by means of a high-frequency inverter, the high-frequency power voltage is lowered by means of a transformer, and, then, the lowered voltage high-frequency power is rectified and smoothed into DC power. The obtained DC current is then applied between the torch electrode and a workpiece. The DC current is detected by a detector, and the inverter is controlled in accordance with the detection value from the detector to maintain the DC current constant. The constant current control provides DC constant current with reduced ripples to be applied between the electrode and a workpiece.
The inventors have recognized that the reduction of ripples in the DC current to be applied between the electrode and a workpiece in order to lengthen the life of the electrode does not necessarily reduce the amount of refractory material consumed. They have found that sometimes the use of DC current containing a larger amount of ripples than ever lengthens the life of electrodes.
The inventors have determined the relationship between the frequency and amplitude of ripples contained in DC current from a power supply apparatus and the amount of refractory material consumed, by experiments. The present invention is based on the results of the experiments, and its object is to provide a power supply apparatus with an increased lifetime.